The development of novel bio-based packaging and barrier materials that can compete with, and replace, conventional non-renewable materials is a great challenge. One of the most severe shortcomings of carbohydrate-based materials is their sensitivity to moisture. At low relative humidity various films made from starches, hemicelluloses, dissolved cellulose and fibrillated cellulose offer excellent barriers against oxygen, but as soon as the relative humidity increases, carbohydrates start to adsorb moisture and this leads to a swelling of the film and a subsequent increase in gas permeability [1-11]. To prevent water vapour transmission, carbohydrate-based films are often considered to be so poor that few researchers even make the effort to measure it, and the amount of experimental data is hence limited [2, 8, 10-13], especially over a broad relative humidity range.